In recent years, an ultrasonic motor which rotates a rotor by utilizing ultrasonic vibration has been proposed and put into practical use. In this ultrasonic motor, progressive wave is generated on the surface of a stator by using a piezoelectric element, and the stator is held in press contact with a rotor, whereby the rotor is moved by virtue of a frictional force therebetween.
For example, Patent Document 1 discloses a multiple-degree-of-freedom ultrasonic motor in which a rotor is held in press contact with a stator by applying a preload to the rotor by means of a spring through a bearing, and, in this state, a drive voltage is applied to a plurality of piezoelectric element plates superimposed one upon the other to generate ultrasonic vibration in the stator, thereby rotating the rotor. Here, the term “preload” means a pressure with which the rotor is pressed against the stator at least in a state in which no electricity is being supplied to the piezoelectric elements.
Patent Document 1: JP 2004-312809 A